Internal combustion (IC) engines generate drive torque by combusting a fuel and air mixture. The drive torque not only provides propulsion force to the wheels but also drives auxiliary engine loads. For example, the drive torque is used to drive loads including, but not limited to, an A/C compressor, a generator or alternator, a coolant pump, an oil pump and the like.
Some internal combustion engines include direct injection control systems that regulate the injection timing of fuel into the cylinders. More specifically, the control system switches engine operation between a high output (HO) mode and a fuel economy (FE) mode based on engine load. In the HO mode, fuel is injected during the intake stroke of the piston to produce a homogeneous combustion charge. The homogeneous charge enables maximum combustion of the oxygen within the cylinder as a result of the fuel distribution throughout the combustion charge.
In the FE mode, fuel is injected during the compression stroke of the piston to form a compact fuel cloud near the spark plug at the time of ignition, providing a stratified combustion charge. Within the stratified combustion charge, a substantial amount of air is not combusted in the combustion process, but is heated and expands to produce power. An improvement in fuel economy is achieved by the reduction of intake stroke pumping work that results from higher intake manifold pressures. Therefore, it is advantageous to stay operating in the FE mode.
During typical engine operation in the FE mode, there are many operating points requiring low engine torque output. At low engine torque output, the engine pumping losses are higher than at higher engine torque output. That is to say, the engine is still not operating as efficiently as desired when operating in the FE mode. Additionally, there are many operating points at or near the transition threshold, which defines the transition point between the HO and the FE modes. Once in the HO mode, hysteresis often delays transition back into the FE mode. Both of these conditions result in missed opportunities to reduce fuel consumption.